Crystal chemistry of minerals of the wöhlerite group from the Los Archipelago, Guinea

• Published in: Canadian mineralogist Vol. 50; no. 3; pp. 593 - 609
• Main Authors: Biagioni, Cristian, Merlino, Stefano, Parodi, Gian Carlo, Perchiazzi, Natale
• Format: Journal Article
• Language: English
• Published: Mineralogical Association of Canada 01-06-2012
• Subjects: accessory minerals; Africa; agpaite; analcime; britholite; britholite group; catapleiite; chemical composition; crystal chemistry; crystal structure; feldspathoid rocks; fluorides; framework silicates; Guinea; halides; hiortdahlite; igneous rocks; lavenite; Los Archipelago; mineral assemblages; Mineralogy; mosandrite; nepheline syenite; nesosilicates; normandite; orthosilicates; peralkalic composition; phosphates; plutonic rocks; polyhedra; serandite; silicates; syenites; villiaumite; West Africa; wohlerite; zeolite group
• ISSN: 0008-4476; 1499-1276
• DOI: 10.3749/canmin.50.3.593

As part of the SYNTHESYS European project, a suite of samples from the nepheline syenites of the Los Archipelago, Guinea, was studied in order to characterize the crystal chemistry of the minerals belonging to the wohlerite group. Chemical and X-ray studies permitted the identification of four different mineral species: lavenite, normandite, wohlerite, and hiortdahlite I. The identification of hiortdahlite II reported by Robles et al. (2001) seems to be questionable. Lavenite and normandite occur in the agpaitic suite, in association with analcime, catapleiite, fluorite, mosandrite, serandite, sphalerite, and villiaumite; wohlerite and hiortdahlite I are associated with britholite-(Ce), magnetite, titanite, and zircon. All the studied lavenite and normandite crystals are intermediate terms in the lavenite-normandite series. Wohlerite shows the highest Mn content ever recorded for this species. Hiortdahlite I is the poorest in terms of both Zr and Na content and the richest in Ca and F among the known occurrences of "hiortdahlite". The crystal structures of these four phases were refined, with R factors ranging from 1.9% (for Mn-rich wohlerite) to 7.3% (for hiortdahlite I). The wide range in Zr-Ti ratios observed in the lavenite-normandite series using new data and all available chemical data, not reported in other Zr-Ti minerals, seems to be related to the contemporaneous substitution of Mn by Ca. The comparison of the geometry of (Nb,Ti)- and Zr-centered polyhedra, including all the available structural studies of the wohlerite group, indicates it is possible to make a distinction between (Nb,Ti)-dominant and Zr-dominant sites in this mineral family.